Generally, with regard to optical disks, such as CD (Compact Disk), MD (Mini Disk), DVD (Digital Versatile Disk) and the like, in a case of producing a master disc of an optical disc, a modulated signal obtained by performing error correction and modulation onto predetermined information data is generated, radiation of a laser beam is controlled in accordance with the obtained modulated signal, and cutting of the master optical disc is carried out. In this way, in accordance with the modulated signal are formed on the master optical disc. With regard to replica optical discs formed in accordance with the master optical disc, the same pits as those on the master optical disc are formed. An optical disk reproducing apparatus irradiates a laser beam onto this optical disc and receives a returning light. Then, the apparatus obtains a reproducing signal whose signal level is variable in accordance with the quantity of the returning light. The apparatus then demodulates the reproducing signal and reproduces information data. In addition, there is a master optical disc producing system which uses a cutting signal inspection apparatus that inspects the cutting of the disc by receiving a returning light signal in order to confirm proper production of the master optical disc.
Now, the returning light signal will be described while taking a master disc producing system in which the optical disc is a CD as an example. FIG. 6 is a view showing a configuration of a master optical disc producing system and a cutting signal inspection apparatus.
Information data, which is recorded on an optical disc master 200, is read out by a reader unit 310 of an EFM (Eight to Fourteen Modulation) signal transmitter 300 and is transferred to a signal processing unit 320. The signal processing unit 320 carries out a necessary signal processing on the information data read out by the reader unit 310, and sends a modulated signal to an EFM output unit 330. The EFM output unit 330 outputs a modulated signal to a laser beam recorder 400 as an EFM signal based on a channel clock. The laser beam recorder 400 irradiates a laser beam onto a master optical disc 410 in accordance with the EFM signal to perform the cutting the optical disc so as to produce the master optical disc. In order to check whether or not the master optical disc is properly produced, a cutting signal inspection apparatus 500 acquires and demodulates a returning light signal from the master optical disc 410 which is being cut, and the cutting signal inspection apparatus 500 carries out a format inspection or verification on the demodulated signal. It is noted that the returning light signal becomes a signal the same as the EFM signal which is a modulated signal.
However, in the conventional cutting signal inspection device or in the conventional optical disc reproducing apparatus, there is a problem that the returning light signal cannot be correctly demodulated in a case where the modulated signal is arbitrarily changed.
An example of a cutting signal inspection apparatus will now be explained. Conventionally, the EFM signal transmitter 300 does not change an EFM signal in particular, and thus, the cutting signal inspection apparatus 500 can demodulate an input signal without any adjustment. However, in recent years, optical discs such as those making a contrast on a signal surface of the disc to have a picture or a symbol on a surface thereof or those having a level and a length of the EFM modulated signal arbitrarily varied for copy protection have been provided. For example, a recording method for recording visibly recognizable letters and figures on an optical disc by controlling formation of pits is suggested in Japanese Patent Application Laid-Open Hei 9-312021. In addition, as suggested in Japanese Patent Application Laid-Open Hei 10-31825, there is a master optical disc producing system in which a timing of a modulated signal is corrected according to a pattern of the modulated signal in order to reduce reproduction error during reproducing an optical disc.
In this way, when the cutting signal inspection apparatus 500 receives a returning light signal from a master optical disc cut by a varied modulated signal the level and the length of which has been arbitrarily varied, since the cutting signal inspection apparatus 500 cannot correctly deal with the varied modulated signal, there occurs demodulation error or demodulation failure at the time of demodulation.